When forest trees are storm-felled it generally happens that trees standing close to electric overhead lines or to telephone lines fall against those lines. The falling tree often does not break the lines, in any case not all the wires thereof, but is halted in an inclined position with its top supported by the wires. In such situations the angle formed between the tree trunk and a horizontal plane is typically between 60.degree. and 80.degree..
It is imperative that storm-damaged areas are cleared as soon as feasible because, especially in winter time, any interruption of the supply of electric power or of the telecommunication network may have a grave impact. In that context, it should be observed that also a very tall, and consequently very heavy, tree may be found in a position where it does not break electric wires or cables but only cause them to sag, namely because most of the weight of the tree is absorbed by an adjacent tree that is still in an upright position. However, even if the wires are not broken when the tree falls, there is a significant risk of it later sliding off its supporting neighbour, continuing its falling movement, and breaking the wires in the process. Consequently, also such a tree must as soon as possible be moved out from its dangerous position.
It should also be noted that, in most cases, it is not possible to relieve the wires by sawing off the trunk at a level rather close above ground because the only result of such an operation is that the base of the trunk slides down from the stump formed and comes to rest on ground. This causes the tilt angle of the trunk to increase a little, i.e., its deviation from the vertical is slightly reduced, but the tree remains hanging with its branches in contact with the wires.
For safety reasons, one cannot resort to the alternative solution of sending somebody climbing up the tree for the purpose of sawing the trunk off at a higher level. That expedient would expose the climber to at least three risks--the risk of being injured by the saw during the work, the risk of being injured if and when the tree falls, and the risk of coming into contact with live electric wires. For the above-mentioned reasons, the only practical solution so far has been to drag the tree root end sidewise to a position where the top is disengaged from the wires and falls to ground. This solution, however, call for use of a tractor or some similar equipment, which is no big problem when the line passes along a road, but may be very difficult and time-consum:ing when it runs through a power line in undulating terrain.
There is another factual circumstance adding to the risks present in clearing work. Such work is generally carried out by linesmen employed by a power distribution company or by a telephone company, as the case may be; a staff category unaccustomed to forestry work. As has been indicated above, a further complication is that such work must as a rule be performed under time stress and, in addition thereto, often under straining environmental conditions such as darkness, strong winds and falling precipitation.